In a number of high-technology fields, as for example the nuclear industry, the aerospace industry, the pipeline industry, and the military, it is frequently required practice to X-ray critical components to screen for defects. The resulting radiographs are sheets of film up to about 14.times.17 inches. Hundreds of thousands of these radiographs are taken each year and stored for record-keeping purposes. Such storage is very expensive because of the sheer bulk of material involved, and the correspondingly large amounts of storage space required. Storage of individual radiographs also presents possible problems of loss of particular records by theft or misfiling.
Several years ago the Nuclear Regulatory Commission (NRC) revised its regulations to authorize nuclear power plants to photoreduce their records instead of storing the originals. To date, however, it has not been possible to realize the very substantial cost savings such photoreductions would represent, because a process and apparatus for accurately photoreducing these industrial radiographs and recovering them undistorted has not been available. The technical specifications for photoreduction and recovery of the full information content of such radiographs are very severe, the NRC regulations and the ASTM standards requiring optical distortion to be not more than 0.1%. This is because it may be necessary at some later time to measure recorded defects and compare the result to the regulatory code standards. The above standard of accuracy also satisfies or exceeds the requirements of all other producers of radiographs.
Although the requirements for accuracy in the photoreduction and reproduction of industrial radiographs are very high, the standards with respect to optical densities are not so severe. It is not necessary that an acceptable photoreduction and reproduction system reproduce optical densities of the original film with an extremely high degree of accuracy. It is, however, necessary that the relative optical densities of different portions of the films be preserved.
The apparatus and procedures employed routinely in the medical field to photoreduce medical X-rays and reenlarge them for later viewing will not serve for the photoreduction and reenlargement of the industrial radiographs which are the subject of the preset invention, because the demands for accuracy in the medical field are much less severe than in the nuclear regulatory field and in other critical industrial applications. As a result, the photographic processes and apparatus employed to photoreduce and subsequently reproduce medical X-rays do not meet the required standards for photoreduction and subsequent reenlargement of industrial radiographs for regulatory purposes.
Accordingly, it would be very desirable to have available a photographic process and apparatus capable of photoreducing industrial radiographs with extremely high resolution and accuracy, and reenlarging the reduced radiographs with essentially no loss in resolution or accuracy, while simultaneously maintaining the relative optical densities of the original radiographs.